Conventional glass sheet heating furnaces include roller conveyors on which glass sheets are heated while conveyed in a generally horizontal orientation through an associated heating chamber. Convection between both top and bottom surfaces of the conveyed glass sheets and the gas within the heating chamber performs a certain extent of the heating. Conduction between heated rolls of the conveyor and the bottom surface of each conveyed glass sheet also provides for the heating. In addition, the preferred construction of these furnaces utilize electric resistance elements both above and below the conveyor rolls such that both the top and bottom surfaces of the glass sheet are also heated by radiation.
Heating of glass sheets to a desired temperature for processing is most preferably performed as rapidly as possible to keep the furnace short to lower heat losses and so as to provide heating with as little distortion as can be achieved. The use of electric resistance elements requires high energy input to bring the furnace up to temperature, requires a long-time interval to effect upward temperature changes and requires high energy input to maintain a given temperature once that temperature is established.